In C++23, you can zip the two arrays, so there's no need to declare an otherwise useless index variable:

int stdSumOfValidDataPoints(const MyData& data)
{
    using namespace std;
    auto&& r = ranges::views::zip_transform(
        [](int data, bool valid){return data * valid;},
        data.dataPoint,
        data.dataPointIsValid
    );
    return accumulate(begin(r), end(r), 0);
}

Unfortunately, the current implementation of zip_transform in GCC doesn't understand that the output length is constant 8, so it does needless cmps every iteration to check if it should stop iterating. (Godbolt)